Organic light-emitting diodes (OLEDs) are a promising technology for flat-panel displays and area illumination lamps. The technology relies upon thin-film layers of organic materials coated upon a substrate. OLED devices generally can have two formats known as small-molecule devices such as disclosed in U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic EL element, and a cathode. The organic EL element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EL layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved. However, the materials comprising the organic EL element are sensitive and, in particular, are easily destroyed by moisture and high temperatures (for example greater than 140 degrees C.).
It has been found that one of the key factors that limits the efficiency of OLED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the OLED devices. Due to the relatively high optical indices of the organic and transparent electrode materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the OLED devices and make no contribution to the light output from these devices. Because light is emitted in all directions from the internal layers of the OLED, some of the light is emitted directly from the device, and some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost in this manner. Moreover, the organic and electrode materials absorb light, further reducing the device efficiency.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from such a device may be emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent cathode layer. Light generated from such an alternative device may be emitted through the top transparent electrode. This is commonly referred to as a top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 1.8-2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. One such technique, taught in US 2006/0186802 entitled “OLED Device having Improved Light Output” by Cok et al, which is hereby incorporated in its entirety by reference, describes the use of scattering layers formed over the transparent electrode of a top-emitter OLED device. It also teaches the use of very thin layers of transparent encapsulating materials deposited on the electrode to protect the electrode from the scattering layer deposition. Preferably, the layers of transparent encapsulating material have a refractive index comparable to the refractive index range of the transparent electrode and organic layers, or is very thin (e.g., less than about 0.2 micron) so that wave guided light in the transparent electrode and organic layers will pass through the layers of transparent encapsulating material and be scattered by the scattering layer.
Transparent conductive electrodes typically employ sputter-deposited conductive metal oxides such as indium tin oxide. Such electrodes suffer from a variety of handicaps, principally inadequate conductivity and, in some circumstances, transparency. In top-emitting device structures, such sputter-deposited layers may also physically damage the underlying organic material layers. Barrier layers providing sputter-deposition protection have been suggested, such as described in WO 97/42666 and U.S. Pat. No. 6,488,555. However, the described barrier layers and sputter deposited electrode layers typically are not sufficiently impermeable to environmental contaminants when employed as the transparent top electrode in a top-emitting device, necessitating the use of additional encapsulating overcoat layers or sealed transparent glass covers, thereby exacerbating problems with light trapping and/or increased costs for such devices.
It is well known that OLED materials are subject to degradation in the presence of environmental contaminants, in particular moisture. Organic light-emitting diode (OLED) display devices typically require humidity levels below about 1000 parts per million (ppm) to prevent premature degradation of device performance within a specified operating and/or storage life of the device. Control of the environment to this range of humidity levels within a packaged device is typically achieved by encapsulating the device with an encapsulating layer and/or by sealing the device, and/or providing a desiccant within a cover. Desiccants such as, for example, metal oxides, alkaline earth metal oxides, sulfates, metal halides, and perchlorates are used to maintain the humidity level below the above level. See for example U.S. Pat. No. 6,226,890 B1 issued May 8, 2001 to Boroson et al. describing desiccant materials for moisture-sensitive electronic devices. Such desiccating materials are typically located around the periphery of an OLED device or over the OLED device itself.
In alternative approaches, an OLED device is encapsulated using thin multi-layer coatings of moisture-resistant material. For example, layers of inorganic materials such as metals or metal oxides separated by layers of an organic polymer may be used. Such coatings have been described in, for example, U.S. Pat. Nos. 6,268,695, 6,413,645 and 6,522,067. A deposition apparatus is further described in WO2003090260 A2 entitled “Apparatus for Depositing a Multilayer Coating on Discrete Sheets”. WO0182390 entitled “Thin-Film Encapsulation of Organic Light-Emitting Diode Devices” describes the use of first and second thin-film encapsulation layers made of different materials wherein one of the thin-film layers is deposited at 50 nm using atomic layer deposition (ALD) discussed below. According to this disclosure, a separate protective layer is also employed, e.g. parylene. Such thin multi-layer coatings typically attempt to provide a moisture permeation rate of less than 5×10−6 gm/m2/day to adequately protect the OLED materials. In contrast, typically polymeric materials have a moisture permeation rate of approximately 0.1 gm/m2 day and cannot adequately protect the OLED materials without additional moisture blocking layers. With the addition of inorganic moisture blocking layers, 0.01 gm/m2/day may be achieved and it has been reported that the use of relatively thick polymer smoothing layers with inorganic layers may provide the needed protection. Thick inorganic layers, for example 5 microns or more of ITO or ZnSe, applied by conventional deposition techniques such as sputtering or vacuum evaporation may also provide adequate protection, but thinner conventionally coated layers may only provide protection of 0.01 gm/m2/day.
WO2004105149 A1 entitled “Barrier Films for Plastic Substrates Fabricated by Atomic Layer Deposition” published Dec. 2, 2004 describes gas permeation barriers that can be deposited on plastic or glass substrates by atomic layer deposition (ALD). Atomic Layer Deposition is also known as Atomic Layer Epitaxy (ALE) or atomic layer CVD (ALCVD), and reference to ALD herein is intended to refer to all such equivalent processes. The use of the ALD coatings can reduce permeation by many orders of magnitude at thicknesses of tens of nanometers with low concentrations of coating defects. These thin coatings preserve the flexibility and transparency of the plastic substrate. Such articles are useful in container, electrical, and electronic applications. However, such protective layers also cause additional problems with light trapping in the layers since they may be of lower index than the light-emitting organic layers.
Among the techniques widely used for thin-film deposition is Chemical Vapor Deposition (CVD) that uses chemically reactive molecules that react in a reaction chamber to deposit a desired film on a substrate. Molecular precursors useful for CVD applications comprise elemental (atomic) constituents of the film to be deposited and typically also include additional elements. CVD precursors are volatile molecules that are delivered, in a gaseous phase, to a chamber in order to react at the substrate, forming the thin film thereon. The chemical reaction deposits a thin film with a desired film thickness.
Common to most CVD techniques is the need for application of a well-controlled flux of one or more molecular precursors into the CVD reactor. A substrate is kept at a well-controlled temperature under controlled pressure conditions to promote chemical reaction between these molecular precursors, concurrent with efficient removal of byproducts. Obtaining optimum CVD performance requires the ability to achieve and sustain steady-state conditions of gas flow, temperature, and pressure throughout the process, and the ability to minimize or eliminate transients.
Atomic layer deposition (“ALD”) is an alternative film deposition technology that can provide improved thickness resolution and conformal capabilities, compared to its CVD predecessor. In the present disclosure, the term “vapor deposition” includes both ALD and CVD methods. The ALD process segments the conventional thin-film deposition process of conventional CVD into single atomic-layer deposition steps. Advantageously, ALD steps are self-terminating and can deposit precisely one atomic layer when conducted up to or beyond self-termination exposure times. An atomic layer typically ranges from about 0.1 to about 0.5 molecular monolayers, with typical dimensions on the order of no more than a few Angstroms. In ALD, deposition of an atomic layer is the outcome of a chemical reaction between a reactive molecular precursor and the substrate. In each separate ALD reaction-deposition step, the net reaction deposits the desired atomic layer and substantially eliminates “extra” atoms originally included in the molecular precursor. In its most pure form, ALD involves the adsorption and reaction of each of the precursors in the complete absence of the other precursor or precursors of the reaction. In practice in any process it is difficult to avoid some direct reaction of the different precursors leading to a small amount of chemical vapor deposition reaction. The goal of any process claiming to perform ALD is to obtain device performance and attributes commensurate with an ALD process while recognizing that a small amount of CVD reaction can be tolerated.
In ALD applications, typically two molecular precursors are introduced into the ALD reactor in separate stages. For example, a metal precursor molecule, MLx, comprises a metal element, M that is bonded to an atomic or molecular ligand, L. For example, M could be, but would not be restricted to, Al, W, Ta, Si, Zn, etc. The metal precursor reacts with the substrate, when the substrate surface is prepared to react directly with the molecular precursor. For example, the substrate surface typically is prepared to include hydrogen-containing ligands, AH or the like, that are reactive with the metal precursor. Sulfur (S), oxygen (O), and Nitrogen (N) are some typical A species. The gaseous precursor molecule effectively reacts with all of the ligands on the substrate surface, resulting in deposition of a single atomic layer of the metal:substrate-AH+MLx→substrate-AMLx-1+HL  (1)where HL is a reaction by-product. During the reaction, the initial surface ligands, AH, are consumed, and the surface becomes covered with L ligands, which cannot further react with metal precursor MLx. Therefore, the reaction self-terminates when all the initial AH ligands on the surface are replaced with AMLx-1 species. The reaction stage is typically followed by an inert-gas purge stage that eliminates the excess metal precursor from the chamber prior to the separate introduction of the other precursor.
A second molecular precursor then is used to restore the surface reactivity of the substrate towards the metal precursor. This is done, for example, by removing the L ligands and redepositing AH ligands. In this case, the second precursor typically comprises the desired (usually nonmetallic) element A (i.e., O, N, S), and hydrogen (i.e., H2O, NH3, H2S). The next reaction is as follows:substrate-A-ML+AHY→substrate-A-M-AH+HL  (2)This converts the surface back to its AH-covered state. (Here, for the sake of simplicity, the chemical reactions are not balanced.) The desired additional element, A, is incorporated into the film and the undesired ligands, L, are eliminated as volatile by-products. Once again, the reaction consumes the reactive sites (this time, the L terminated sites) and self-terminates when the reactive sites on the substrate are entirely depleted. The second molecular precursor then is removed from the deposition chamber by flowing inert purge-gas in a second purge stage.
In summary, then, an ALD process requires alternating in sequence the flux of chemicals to the substrate. The representative ALD process, as discussed above, is a cycle having four different operational stages:
1. MLx reaction;
2. MLx purge;
3. AHy reaction; and
4. AHy purge, and then back to stage 1.
This repeated sequence of alternating surface reactions and precursor-removal that restores the substrate surface to its initial reactive state, with intervening purge operations, is a typical ALD deposition cycle. A key feature of ALD operation is the restoration of the substrate to its initial surface chemistry condition. Using this repeated set of steps, a film can be layered onto the substrate in equal metered layers that are all identical in chemical kinetics, deposition per cycle, composition, and thickness. However, such processes are expensive and lengthy, requiring vacuum chambers and repeated cycles of filling a chamber with a gas and then removing the gas.
ALD and CVD processes as conventionally taught, typically employ heated substrates on which the materials are deposited. These heated substrates are typically at temperatures above the temperatures organic materials employed in OLED devices can tolerate. In addition, the films formed in such processes may be energetic and very brittle, such that the subsequent deposition of any materials over the films destroys the film's integrity.
Thus, a need exists for an OLED architecture that decreases damage due to electrode deposition, increases lifetime, and improves the efficiency of light emission.